A power circuit breaker or circuit interrupter can be divided into three major components:
1. arc-interrupting and current-carrying contacts; PA1 2. entrance bushings; and PA1 3. an operating mechanism (operator). PA1 1. Close contacts against the magnetic force resulting from the maximum momentary-current rating and against any spring pressure that may be used as an opening energy source within a time range of 10 to 15 A.C. cycles (0.166 to 0.250 seconds); PA1 2. Allow contacts to open against inertial, frictional, and pressure forces at a speed that will result in 2 to 8 cycle (0.033 to 0.133 seconds) interruption of current and meet the prescribed velocities and time position requirements of the associated interrupter; PA1 3. Allow contacts to open and then reclose in a total time of 15 to 30 cycles (0.25 to 0.50 seconds); PA1 4. Perform at least five closing and opening operations without the energization of a prime mover; PA1 5. Operate in climates having temperature variations of as much as -30.degree. C. to +40.degree. C. and in locations that vary from very dry and dusty to very hot and humid; PA1 6. Be able to stand idle for periods up to a year, then operate the very first time at full speed and power; PA1 7. Require low control currents; PA1 8. Be economical to produce and maintain; and PA1 9. Require little if no periodic maintenance for reliable operation. PA1 1. solenoid, PA1 2. spring, PA1 3. pneumatic, and PA1 4. hydraulic.
Each of these components is vital to the operation of a circuit breaker; weakness in any one will result in unsuccessful breaker operation. After a circuit breaker has been placed in service, practical experience has shown that the operating mechanism requires the most attention.
The basic function of the operating mechanism is to open and close the breaker contacts. This, by itself, is a comparatively simple task. However, circuit breaker operating mechanisms are usually specified to meet the following design criteria:
The internal mass of the contacts, arc extinguishing gas compressive elements, connecting linkages, and the like in a large circuit breaker is considerable. A relatively large amount of energy is required to start any movement--especially in the short interrupting time required of a modern circuit breaker.
At present, there are four types of operating mechanisms (operators) used to operate high-voltage circuit breakers:
The solenoid operator uses a large electric solenoid as a source of energy for closing. The solenoid requires large current inputs and is relatively slow. Generally used on small breakers, the solenoid operator is losing its importance. Although a solenoid operator requires no charge-up time, it cannot operate after loss of control power.
A spring operating mechanism stores energy in springs that are compressed by action of an electric motor. It is generally used for small distribution breakers where it is replacing the solenoid operator.
All the large circuit breakers requiring high-speed interrupting and high-speed reclosing use either a pneumatic or a hydraulic operating mechanism. These two types are similar enough to be discussed at the same time, using comparison as a means of pointing out differences.
The basic source of stored energy is the same for both the pneumatic and hydraulic operator: a compressed gas. The pneumatic operator compresses air by means of an electric, motor-driven, air compressor; the hydraulic operating mechanism operates with a gas compressed in an accumulator by means of an electric, motor-driven, oil pump. In a hydraulic operator sufficient energy is stored in an accumulator for several trip cycles. Unfortunately, the inherent limitations of accumulators are not taken into consideration when many hydraulic operating mechanisms are designed.
One major difference between pneumatic operators and hydraulic operators is the manner in which the gas is used: pneumatic operators are low-pressure (150 to 300 psi), high-volume gas systems; hydraulic operators have high-pressure (1500 to 5000 psi), low-volume gas systems. A comparison of the time required to charge the two gas systems from zero to full operating pressure shows that, depending on the size of the operating mechanism, it takes 40 to 60 minutes to charge a pneumatic system, while the gas system of a hydraulic accumulator is charged in about 8 to 15 minutes.
Another major difference between the two systems is that the hydraulic operator is basically a closed system; that is, in the absence of leaks gas never leaves the accumulator and the oil travels from a sump into the accumulator and back to the sump. Thus, there is little chance for foreign contamination. The pneumatic operator on the other hand, takes air from the atmosphere, compresses it, and then expels it back into the atmosphere. During compression, the moisture is condensed and accumulates in the gas system. This moisture must be removed regularly, depending upon atmospheric conditions. In cold climates, it must be kept from freezing or the valves in the operating mechanism may become inoperative. In warm, humid climates, water is condensed on metal surfaces similar to that often seen coming out of air conditioners. Naturally, this leads to internal corrosion. The moisture problem is probably the biggest source of trouble for maintenance people. For these two reasons alone, hydraulic accumulators are by far the preferred source of fluid energy to operate large circuit breakers.
In addition to a source of fluid, hydraulic accumulators also provide several other useful functions. Since all hydraulic systems eventually develop leaks, the accumulator compensates for external leakage and thereby maintains hydraulic pressure within an acceptable range for long periods of time. In the same manner, the accumulator compensates for thermal expansion and contraction of the liquid due to variations in temperature. Finally, the accumulator also dampens pressure surges caused by the electrically driven hydraulic pump when it is cycled on and off. This prevents damage to the components of the hydraulic system caused by vibration and shock. Not all accumulator designs (i.e. spring operated, gravity operated) are especially adopted to use as an emergency source of hydraulic power for a circuit breaker. Gas-operated accumulators are preferred.
Gas-operated accumulators are often referred to as pneumatic or hydropneumatic accumulators. Gas-operated accumulators are classified as either nonseparator or separator types. In the nonseparator type accumulator, no means are provided for separating the gas from the liquid. In the separator type of gas-operated accumulator, a dividing means or separator is provided to separate the gas from the liquid. Three types of separators are used: a bladder or bag; a diaphragm; or a piston (e.g. U.S. Pat. No. 3,136,340).
Unfortunately, the rubber diaphragm in the diaphragm accumulator and the soft rubber bladder or bag in the bladder accumulator leaks gas by osmosis. This leakage can amount to almost 10% per year. Thus, the gas space in each of these accumulators must be periodically charged. This is an unnecessary nuisance and inconvenience. It is also possible for the bladder to rupture. In addition, these accumulators must be visually examined periodically for indications of hydraulic leaks in that the soft rubber bladder or diaphragm can easily leak. If the separating material fails, the accumulator must be removed, repaired, and reinstalled. This is a relatively complicated and time consuming task.
Before a bladder or diaphragm can be repaired, all internal pressure must be relieved. In addition, the air or gas in the accumulator must be discharged. It should be appreciated that the time that an electrical distribution system is placed out of service is inevitably long. Thus, the preferred accumulator in a hydraulically operated circuit breaker is a piston type gas-operated accumulator. Unfortunately, piston accumulators as used in circuit breakers have not been used to their greatest advantage. Moreover, some of their inherent limitations are frequently compensated for by other components. An innovative approach to the manner in which conventional accumulators can be used would go far towards improving the overall reliability of hydraulically operated circuit breakers.